A variety of devices, using various materials and processes, provide noise abatement performance for automobiles: (1) plastic injection molded covers and those with added sound absorbers (2) fiberglass, (3) die-cut or molded (cast) foam and (4) high density foam.
Plastic injection molded covers provide aesthetic value, but typically do not provide sound absorption performance. Plastic injection molded covers can be fitted with sound absorbers (die-cut foam, molded (cast) foam, fiberglass), but this adds multiple parts in assembly and added expense.
Fiberglass provides level of sound absorption, but not high quality appearance. A commonly-used sound absorption material is specialized acoustic fiberglass.
Die-cut or molded (cast) foam provides a level of sound absorption, but not quality surface appearance. As noted, this can be used alone, but also in combination with other components.
The automotive industry uses foam material for noise abatement. Polyurethane foam is known for acoustical panels for sound barrier dampening and sound absorbsion. It reduces passenger compartment noise including in the engine compartment with polyurethane foam technology, including a sound absorption panel as a shaped component adapted to an engine compartment. Published Patent Application 2007/0287001 discloses a sound-absorbing compartment lining having a foam material layer and fiber layer made from mineral fibers. Another example for an automotive engine hood is as shown in U.S. Pat. No. 4,584,232. It discloses a sound absorbing member including a fiber grid insert disposed between and bonded to two open cell foam panels. Similarly, U.S. Pat. No. 4,288,490 discloses an engine hood lining for automotive vehicles having a continuous sheet-like body that is formed of a closed-cell polyethylene foam material and is shaped to fit the inside contour of a hood of the vehicle.
High density (i.e. 19 PCF) micro-cellular foam provides barrier/damper performance and good appearance, but not a high level of sound absorption performance.
Such a cover for the top of an engine is produced in Germany for the Mini Cooper, using high density, micro-cellular foam at approximately 15-19 PCF (240-300 kg/m3), Carcoustics (of Germany) is a Tier-2 supplier of engine cover for the Mini Cooper. Baur Formschaumtechnik (of Mindelheim Germany) is the Tier-3 molder of part. High density foam does not provide a high level of sound absorption performance. As such, certain foam products are used in close association with the engine.
U.S. Pat. No. 4,863,791 discloses sound absorption in foam core panels including a method of manufacture of a foam core and vehicle trim panels made from such board. A foam board having a cell structure including a structure with large cells in a center portion is initially formed. This board is skived into two separate panels, each having one surface with relatively large cell structure and wherein said cells have been opened by the skiving process. This surface acts to promote enhanced sound absorption from the foam core board. This board may be laminated with appropriate decorative material and molded to make contoured resilient vehicle trim panels.
Current designs contain multiple parts or components for equivalent appearance and noise abatement performance.